An auxiliary power unit (APU) on an aircraft may be a gas turbine engine configured to produce electrical and pneumatic power. The APU may provide power to aircraft systems to either supplement or substitute power generated by the main engines. An APU may be a small gas turbine engine that is mounted within the aircraft fuselage and which draws air through inlets defined within the outer skin of the aircraft. APU may also be used to power aircraft such as unmanned drones or ballistic missiles.
A gas turbine engine typically includes a compressor section, a combustor section, and a turbine section. In general, during operation, air is pressurized in the compressor section and is mixed with fuel and burned in the combustor section to generate hot combustion gases. The hot combustion gases flow through the turbine section, which extracts energy from the hot combustion gases to power the compressor section and other gas turbine engine loads.